The best advice I've ever been given regarding SEO is just to think like a machine reading your webpage, instead of a human.

Basically:

Don't over-use javascript links for for other pages, makes it hard for a crawler bot to both identify and follow the links

What Dylan said, make all links clearly labelled with good titles and/or images, ALT labels for images, etc.

Try to avoid ajax for things other than grids or similar, if your entire site is just http://www.domain.com/ and you use ajax for anything else, how can a search engine link to any particular bit, much less find it?

Although people refer to it as SEO, I think to think of a lot of "SEO" techniques as just good practice.

A few examples:

Having sensible URL's, such as /articles/2008/aug/how-to-build-a-nice-desk/ is not only nice for Google (as it's loaded with keywords) but it's easy for your visitors to write down or repeat over the phone. It's sure nicer than /index.php?module=articles&article=128410.

Things such as frames and excess use of Javascript need to be used carefully to make a users experience easier - if you give the user a good experience (such as real hyperlinks or history-friendly ajax) the search engines will also have an easier time finding your content.

Having semantically correct HTML not only helps with SEO, but helps with cross-browser compatibility.

Search engines love fresh content... so do your users.

The list could go on, but general SEO techniques seem to say to me "Design a site that's easy to use, follows standards, and that has fresh content and you'll probably get decent linkbacks and decent search engine ranking".

It's all about having content that compels the audience you seek to come to your website. Basically that's the gist of Google's Search Engine Optimization Guide. Most SEO magic is an attempt to make a contentless site appear to have relevance.
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Fiasco LabsMar 3 '12 at 3:59

Not always. A lot of SEO magic is also actually helping the search engines find the content you have. Great content will draw visitors, but if you apply just a few tweaks and improvements to help Google find your great content you'll score a lot better.
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Stephan MullerMar 16 '12 at 9:06

Are there any books or do these date so quickly that they are obsolete?

I hear the same SEO arguments now that I did four years ago. Most of the advice I would say is timeless.

NetMechanic tips haven't been updated since 2006, but even the articles dating back to 1998 are still relevant today.

Is it event worth learning the techniques as search engines change their algorithms so regularly?

I would say it's definitely worth learning some of the techniques. Often the tips apply as much to search engine optimisation as they do to website accessibility. In my experience search engines change only to favour websites that are honest and use markup intended for humans (meaningful alt tags, rather than alt tags or hidden text crammed with spammy keywords) and to punish those using "Black Hat SEO" who do have to change their techniques as search engines evolve.

I've yet to come across any reputable SEO books, but there are approaches to SEO worth learning.

First off, the algorithms that Google/Yahoo/Bing! use are irrelevant. Modern search engines have advanced to the point that they behave mostly as they should (despite the best efforts of the SEO industry). Beyond the fundamental principles of PageRank, no one really knows how Google's algorithms work. There are press releases about new search features and fixes to previous search problems, but the details of each ranking system's implementation is mostly speculative. Pretty much all of the information published by 3rd parties is based on finding vague statistical correlations and speculating on these "ranking factors".

So if you take the blackhat approach to SEO, i.e. trying to game the ranking algorithms, you won't find much concrete information to go on. And the frequent algorithm changes/fixed that are designed to combat blackhat techniques mean that any effective technique will just backfire in the long run.

But if you take the whitehat approach to SEO recommended by Google and other search engines, then you don't need to worry about the lack of detailed ranking algorithm information or the constant algorithm changes. That's because whitehat SEO really is just trying to make a useful and accessible website. This is much harder to do if you're a fly-by-night operation attempting to make a quick buck without providing anything of value to users. But if you started a site based on the genuine desire to provide useful information or services, then SEO will just be a natural side-effect of improving your website for users.

This also explains the lack of reputable books on SEO. Most SEO practices really fall into usability and accessibility best practices (e.g. using clean URLs, proper semantic markup, RESTful architecture, etc.), while others are just commonsense (like using good grammar/spelling, providing unique/useful/relevant content, etc.). So get a book on usability and optimizing web navigation, as well as study up on semantic HTML standards (like the proper use of meta and link tags) and Web Content Accessibility Guidelines. This type of SEO requires you to be invested in the long-haul, but it guarantees good results no matter how search engines adapt and evolve over time.

Lastly, you should be warned that the majority of the SEO industry that claims to be "whitehat" is actually in the greyhat category. SEO, as a marketing-based industry, is rather devoid of morals. Really, the majority of SEO professionals seem to eschew well known blackhat techniques like link farming and keyword stuffing simply because the search engines have wised up to them (most of the old-timers themselves practiced these techniques before they were penalized by search engines and labeled blackhat).

So be careful about whom you listen to. Even if something doesn't hurt your search ranking, it could still permanently damage your online reputation. This is especially pertinent considering that most "whitehat" SEOs (including the top SEO sites) still consider content-spinners and blog spamming acceptable forms of SEO. They might add a little disclaimer like "be sure your comment is useful and don't post to the same blog too many times in a row", but they still encourage webmasters to actively seek out unprotected blogs that don't inject nofollows into links (e.g. on "high value" .edu sites) and post comments linking to yourself to game the PageRank system. And in that respect, SEO isn't really worth investing in explicitly for legitimate businesses and site owners. These types of practices that are pursued solely for the purpose of improving one's search ranking without improving user experience actually de-optimize search engine results and put you on the same level as spammers.

In short, yes! Ignore SEO at your peril, especially if you are creating new websites with little or no domain history. My advice is get the basics down first then worry about all the more trivial aspects at a later stage. I recently read this article on Minimum Viable SEO which is sensible advice.

If you want more in depth reading and want to keep up with the changes you might want to check out this
blog. It is the blog of Matt Cutts, a Google employee and head of the Webspam team. Reading through many of his posts will help you keep in line with the recommended practices to stay on the good side of Google. There is no "must read" book because the algorithm of search is organically changing, sometimes on a monthly basis (May Day, Google Caffeine etc).

Might I also recommend http://www.seo-theory.com/ as a great source of material and articles (I have no affiliation with this website, by the way).